The present invention relates to a semiconductor laser device, and more particularly to a semiconductor laser device which is suitable for serving as a light source for an optical disk.
A semiconductor laser for an optical disk is required to have a low noise characterisitic. A gain-guided, multi-longitudinal mode laser has such a characteristic (refer to J. Appl. Phys., Vol. 56, 1984, pages 3116 to 3120). In a gain-guided semiconductor laser, however, astigmatism is unavoidable. Thus, it is difficult to focus a laser beam into a small spot. In the semiconductor laser described in the above-referenced publication, the width of a current injection region is varied within a laser crystal, to reduce the astigmatism. However, it is impossible to reduce the astigmatism in a marked degree by the above structure.
Further, a semiconductor laser has been proposed in which an end surface having the form of a curved surface is formed to reduce the astigmasism (refer to Appl. Phys. Lett., Vol. 46, 1985, pages 218 to 220). In this semiconductor laser, however, a stripe-shaped active region has a fairly large width, and hence a far field pattern is not a single lobed pattern. Moreover, a fairly large amount of astigmatism is generaged. This is because, when the stripe-shaped active region is made large in width, it is impossible to control the transverse mode of laser oscillation.
As mentioned above, according to the conventional structures, it is impossible to reduce astigmatism which is generated inevitably in a multi-longitudinal mode laser, to a marked degree and to focus the laser beam into a small spot.